In the field of semiconductor processing, photolithography is a widely employed technique to “pattern”, i.e. define a profile in one or more layer of semiconductor material, a semiconductor wafer. Using this technique, hundreds of Integrated Circuits (ICs) formed from an even larger number of transistors can be formed on a wafer of silicon. In this respect, for each wafer, the ICs are formed one at a time and on a layer-by-layer basis.
For about the last four decades, a photolithography apparatus, sometimes known as a cluster or photolithography tool, has been employed to carry out a photolithographic process. The cluster comprises a track unit that prepares the wafer, including providing layers of photosensitive material on the surface of the wafer prior to exposure to a patterned light source. To expose the wafer to the patterned light source, the wafer is transferred to an optics unit that is also part of the cluster. The patterned light source is generated by passing a beam of light through a chrome-covered mask, the chrome having been patterned with an image of a given layer of an IC to be formed, for example, transistor contacts. Thereafter, the wafer is returned to the track unit for subsequent processing including development of the layers of photosensitive material mentioned above.
The wafer, carrying the layers of photosensitive material, is supported by a movable stage. A projection lens focuses the light passing through the mask to form an image on a first field over the layers of photosensitive material where an IC is to be formed, exposing the field of the layers of photosensitive material to the image and hence “recording” the pattern projected through the mask. The image is then projected on another field over the layers of photosensitive material where another IC is to be formed, this field over the layers of photosensitive material being exposed to the projected image, and hence pattern.
The above process is repeated for other fields where other ICs are to be formed. Thereafter, the wafer is, as mentioned above, returned to the track unit, and the exposed layers of photosensitive material, which become soluble or insoluble through exposure depending upon the photosensitive materials used, are developed to leave a “photoresist” pattern corresponding to a negative (or positive) of the image of a layer of one or more ICs to be created. After development, the wafer undergoes various other processes, for example ion implantation, etching or deposition. The remaining layers of photosensitive material are then removed and fresh layers of photosensitive material are subsequently provided on the surface of the wafer depending upon particular application requirements for patterning another layer of the one or more ICs to be formed.
In relation to the patterning process, the resolution of the scanner impacts upon the width of wires and spaces therebetween that can be “printed”, the resolution being dependent upon the wavelength of the light used and inversely proportional to a so-called “numerical aperture” of the scanner. Consequently, to be able to define very high levels of detail a short wavelength of light is required and/or a large numerical aperture.
The numerical aperture of the scanner is dependent upon the product of two parameters. A first parameter is the widest angle through which light passing through the lens can be focused on the wafer, and a second parameter is the refractive index of the medium through which the light passes when exposing the layers of photosensitive material on the wafer.
Indeed, to provide the increased resolution demanded by the semiconductor industry, it is known to reduce wavelengths of light used whilst also making lenses bigger to increase the numerical aperture. However, the limits to which the wavelengths of light used can be reduced are rapidly being reached, since wavelengths of less that 157 nm are absorbed by the lenses used.
Additionally, the above-described scanner operates in air, air having a refractive index of 1, resulting in the scanner having a numerical aperture between 0 and 1. Since the numerical aperture needs to be as large as possible, and the amount the wavelength of light can be reduced is limited, an improvement to the resolution of the scanner has been proposed that, other than by increasing the size of the lens, uses the scanner in conjunction with a medium having a refractive index greater than that of air, i.e. greater than 1. In this respect, the more recent photolithographic technique proposed, employing water and known as immersion lithography, can achieve higher levels of device integration than can be achieved by air-based photolithography techniques.
Therefore, scanners employing this improvement (immersion scanners) continue to use low wavelengths of light, but the water provides a refractive index of 1.4 between the lens and the wafer, thereby achieving increased resolution through increasing the numerical aperture of the immersion lithographic apparatus by a factor of 1.4.
Further, the refractive index of the water is very close to that of fused silica from which some lenses are formed, resulting in reduced refraction at the interface between the lens and the water. The reduced refraction allows the size of the lens to be increased, thereby increasing the numerical aperture further.
Whilst it appears that immersion lithography can achieve wafer throughputs comparable to air lithography, difficulties exist when introducing water between the lens and the wafer. One way of placing water between the lens and the wafer involves injecting a small film of water between the wafer and the lens, the film covering a field over the surface of the layers of photosensitive material where a given IC is to be formed, rather than the entire wafer.
However, by placing water between the lens and the wafer, and indeed in contact with an uppermost surface of the layers of photosensitive material, defects can be introduced. Such defects, or contaminants, when in the focal plane of the optics unit (sometimes known as a “scanner”) affect the ability of an immersion lithography apparatus to print defect-free lines and spaces. In this respect, defect levels can be affected by particle impurities in the water, temperature variations of the water, and thickness uniformity of the water layer. Additionally, bubbles can form in the water layer, which can scatter the light from the lens, causing blurring and distortion of the projected image. Possible sources of bubbles are air dissolved in the water, air introduced into the water as it is ejected from nozzles, air introduced through turbulence caused by movement of a stage or projection system, and/or “out gassing”, the egress of gas from the photosensitive material as a result of impurities or simply the components of the photosensitive material. Whilst so-called “degasification” can remove bubbles caused by air dissolved into the water and careful nozzle design can eliminate the nozzle-induced bubbles, air trapped on the uppermost surface of the layers of photosensitive material can also create bubbles as the water flows over the surface of the wafer.
One proposed solution to avoid imaging problems caused by bubbles adjacent the surface of the wafer is to apply a top anti-reflection coating over the layers of photosensitive material prior to placing the wafer in the immersion lithography apparatus, thereby creating a barrier between bubbles and the uppermost surface of the layers of the photosensitive material. However, such a solution, whilst keeping bubbles further from the focal plane, i.e. the uppermost surface of the layers of photosensitive material, serves to increase the focal distance and hence reduce the resolution that can be achieved.